Color process involving change in isoelectric point of gelatin



Nov. 14, 1950 D. DEAN ET AL 2,529,922

COLOR PROCESS INVOLVING CHANGE IN ISOELECTRIC POINT OF GELATIN Filed Feb. 2a, 1948 EXPOS E DEVELOPIN NONHARDENING DEVELOPER DEVELOP IN PYRO LOWER LP. AND

STOP BATll ExPosE AND 8 AND HARDEN DEVELOP m PYRO HARDENED. N LOWER |-P. AND

HARDENED ACID DYEBATH HAVING HAEDEN pH BETWEEN LP. 0F 19 Two IMAGES 22 HARDENED DYED GELATIN ACID DYE BATH HAVING l PH BETWEEN l-P. 0F

23 Two IMAGES ED T W. PMM

ATTORNEYX Patented Nov. 14, 1950 COLOR PROCESS INVOLVING CHANGE IN ISOELECTRIC POINT OF GELATIN David Dean and Robert C. Houck, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application February 28, 1948, Serial No. 12,018

5 Claims. (01. 95-88) This invention relates to color photography and particularly to a color process in which use is made of the differential dyeing properties of gelatins having different isoelectric points.

In photographic color processes of the imbibition transfer type, it is necessary to differentiate between the image and non-image portions by making a relief matrix or by relying upon the differential hardening of the gelatin as in planographic processes. These methods are sometimes undesirable Where the purpose is to secure a differential dyeing of a gelatin layer according to an image formed therein.

It is therefore an object of the present invention to provide a novel color process which depends upon the differential dyeing properties of gelatins having different isoelectric points. A further object is to provide a method for varying the isoelectric point of a gelatin emulsion layer in accordance with an image formed in the layer. A still further object is to provide a novel imbibition transfer color method. Other objects will appear from the following description of our invention.

These objects are accomplished by forming an image in a gelatino-silver halide emulsion layer and changing the isoelectric point of the gelatin differentially in accordance with the image formed in the layer, and dyeing the gelatin layer having images of different isoelectric points, with a dye solution adjusted to a pH between the isoelectric points of the two images.

The accompanying drawing shows in sectional view photographic elements at different stages in two modifications of our process.

The isoelectric point of gelatin is defined as the pH at which it is ionized equally as an acid and as a base. It may also be defined as the pH at which gelatin carries a like number of positive and negative charges and therefore does not move under the influence of an electrical potential. When the pH of the solution is increased or decreased beyond the isoelectric point by adding alkali or'acid, respectively, the gelatin becomes charged negatively or positively respectively. The amount of acid or basic dye which is taken up by gelatin depends on the isoelectric point of the gelatin and the relationof the pH of the dye bath to the pH of this isoelectric point (Sheppard, Houck, and Dittmar, Journal of Physical Chemistry, vol. 46, 1942, page 158). This action may be explained as follows.

When an acid dye is used to dye gelatin at a pH below the isoelectric point of the gelatin, the color is in the dye anion which is negatively charged. If the pH of the dye bath is decreased below that of the isoelectric point of the gelatin, the gelatin becomes positively charged. This p0sitively charged gelatin combines electrostatically with the negatively charged dye anion and thus becomes dyed. The amount of dye taken up by the gelatin increases as the pH is lowered, the

charge on the gelatin becoming more positive as the pH is lowered.

If an acid dye is used at a pH greater than the isoelectric point of the gelatin, both the gelatin and the dye anion are negatively charged. There is therefore no attraction between the gelatin and the dye anion and no dyeing occurs.

In the case of basic dyes, the color is in the cation of the salt and is therefore positively charged. When the pH of a basic dye bath is adjusted to a pH below the isoelectric point of the gelatin, the gelatin is positively charged but the color cation is likewise positively charged and there is no attraction of the gelatin for the dye cation. For this reason, no dyeing occurs with basic dyes at a pH below that of the isoelectric point of the gelatin.

When a basic dye is used to dye gelatin at a pH above the isoelectric point of the gelatin, the gelatin is negatively charged and the color cation is positively charged. There is therefore an attraction between the gelatin and the dye cation and dyeing occurs with basic dyes.

This may be summed up as follows for gelatin and pure acid or basic dyes.

(1) Gelatin is dyed with acid dyes below the pH of its isoelectric point.

(2) Gelatin is not dyed with acid dyes above the pH of its isoelectric point.

(3) Gelatin is not dyed with basic dyes below the pH of its isoelectric point.

(4) Gelatin is dyed with basic dyes above the pH of its isoelectric point.

The pH of the isoelectric point of gelatin may vary over a wide range from about 3.8 to 9 depending upon the method of manufacture and subsequent treatment of the gelatin. Ordinarily, lime process gelatin has an isoelectric point fairly well defined in the regions of pH 4.8 to 5. Acid process gelatin has an isoelectric point in the region of pH '7 and upwards. Such gelatin and a method of producing it are described in Sheppard and Hudson U. S. Patent 2,101,877, granted December 14, 1937.

Other methods of obtaining gelatins of different isoelectric points are (1) Treatment of a gelatin of I. P. about pH 01 above with nitrous acid. This treatment lowers the isoelectric point. See Sheppard and Hudson U. S. application Ser. No. 631,216, filed November 27, 1945, now abandoned.

Treatment of gelatin of I. P. about pH 4.8 or above with an acid chloride. This treatment lowers the isoelectric point, the extent of the lowering depending on the conditions used (pH, temperature, etc.).

The dyeing properties of gelatins described above apply to all types of gelatin regardless of the pH of their isoelectric points.

It is apparent from the above considerations that if in the treatment of a gelatin layer, the isoelectric point is lowered in certain areas, part of the surface being at one isoelectric point and part at another, then diiferential dyeing can be obtained through the use of a dye bath of the proper pH. Depending upon the type of dye and the pH of the bath with relation to the isoelectric points of the two areas, one of the areas can be dyed without dyeing the other area.

We have found that if a gelatino-silver halide emulsion is exposed to form a latent image and is developed with a tanning developer such as pyrogallol, or hydroquinone without sulfite, areas of lowered isoelectric point will be formed as a result of the reaction of the gelatin with the oxidation products of the developer. These oxidation products apparently react with the amino groups of gelatin and lower the isoelectric point where development takes place, at the same time hardening the gelatin in those regions. For purposes of our process, the lowering of the isoelectric point is the important factor with respect to the differential taking up of dye and the fact that tanning or hardening simultaneously occurs is incidental. The advantages of this method of difierential dyeing will be more apparent from the following description of our process.

From a set of three color separation black and white positives correct exposures were made onto three pieces of gelatino-silver halide relief film, the exposure preferably being made through the base. These films were then developed together for five minutes at 70 F. in the following developer.

Solution A Grams Pyrogallol 6 Ascorbic acid 1 Potassium bromide 2 Water to 1 liter Solution B.

Potassium carbonate 20 Water to 1 liter 7 For use, 1 part of Solution A was mixed with two parts of Solution B.

The development was followed by treatment in a stop bath containing 48 cc. of 28% acetic acid per liter of water solution for 1 minute after which the darkroom lights were turned on. At this point there appeared a'negative silver image and the gelatin in the area of this image had had its isoelectric point lowered by the developing action. In the area wherev no development occurred, no'isoelectric point change had taken place.

The films were then treated for five minutes in a hardening bath containing 35 grams of chrome alum per liter of water solution, to give the undeveloped portions of the gelatin layer sufficient hardness to withstand treatment in subsequent steps of the process. The three films were then placed in an acid fixing bath of the 4 following composition for a sufiicient time to clear the silver halide from the undeveloped area of the layer.

Grams Sodium thiosulfate 240 Sodium sulfite (desiccated) 15 Acetic acid (28%) 47.5 Boric acid crystals 7.5 Potassium alum 15 Water to 1 liter After washing the films for 15 minutes, they were placed in their respective dye baths, the red separation film being placed in the cyan dye bath, the green separation film in the magenta dye bath and the blue separation film in the yellow dye bath. The following dyes were used:

Cyan dye Sodium salt of 4,8-diamino-1,5-dihydroxyanthraquinone-2,6-disulphonic acid (C. I. 1054) HzN OH I C O SOaNa NaOa O NHz Magenta dye t HNQQ NH4O3S l-carbethoxy-G (4-chloro-2-sulfobenzamido -3-methyl-3- azabenzanthrone ammonium salt Yellow dye The dye solutions were made to a strength of 0.2 gram of dye per liter of distilled water and no buffers were used. The dye solutions were then adjusted to a pH of 4.7 and the films were placed in the dye baths for 10 minutes.

With the isoelectric point theory of dyeing in mind, the reason for the selection of a pH of 4.7 for the dye bath is apparent. .The original gelatin of the films had an isoelectric point at a pH of about 4.8 to 5, the usual isoelectric point of lime process gelatin. Treatment of the films in the pyrogallol developer lowered the pH of the isoelectric point in the developed region, to about pH 4.0. The dyes used were acid dyes which dye gelatin at a pH below the pH of its isoelectric point but not at a pH above the isoelectric point. Since the isoelectric point of the gelatin was lowered by the development to a point below that of the pH of the dye baths, the gelatin in the developed region will not be dyed. In the undeveldyeing occurs in this region. Both regions of f'the layer are hardened so that dyeing does not depend upon differential hardening.

After treatment in the dye bath, the films were rinsed in tap water to remove surface dye and were squeegeed onto a mordanted transfer paper in succession and in register, a transfer time of about 5 minutes being allowed for each matrix. A full color reproduction was produced in the transfer blank.

- In place of the relief or matrix films anda gelatin with an isoelectric point at pH 4.8 to 5, it is preferable to use a special emulsion made from acid process gelatin. This gelatin has an isoelectric point in the region of pH '7 to 8 and a greater diflerence between the isoelectric point pH of the original gelatin and that of the developed region can therefore be obtained. When such gelatin is used, the development reduces the pH of the isoelectric point from 8 to about 5. This allows a better dyeing differentiation to be made.

Using a sensitive material of this type, that is, in which the gelatin has an isoelectric point at about pH 8, three exposures were made through red, green and blue filters using a multi-color original image. After exposure, these films were developed togetherfor' five minutes in the following developer, one part of Solution A being mixed with two parts of Solution B.

Solution A Pyrogallol grams 6 Ascorbic acid do 1 Potassium bromide l; do 2 Benzothiazole (0.2% solution) cubic centimeters 40 Water to l'liter Solution B Potassium carbonate grams 200 Water to 1 liter Development was followed by 1 minute treatment in the acid stop bath referred to above. The films were then placed directly in a clearing bath containing 200 grams of sodium thiosulfate per liter of water. After three minutes treatment in this bath, the films were washed for min-' utes and allowed to dry. Additional hardening may also be given the film prior to drying.

The three films were then placed separately in the dye baths referred to above. In these cases, the dye baths contained 0.4 gram dye and 4.0 grams disodium phosphate (N'a2HPO4-12HzO) per liter and were adjusted to pH 7.8 with-phosphoric acid. The three films were treated in their respective dye baths for five minutes followed by a -second rinse in tap water to remove surface dye. After successive transfer of the dyes from the matrices to a piece of mordanted transfer paper, a full color print was obtained.

We have described obtaining positive dye images by exposure'of the matrix to a positive color separation or positive colored original. It is also possible to obtain negative images from positive originals or positive images from negative originals by first developing the exposed films in a non-hardening developing bath, washing, reexposing the undeveloped areas followed by developing in a pyrogallol developer.

According to this method, the films were eX- posed to a negative multi-color image through color separation filters and were developed for four minutes in a developer of the following c n rposition: v

Grams Monomethyl-p-aminophenol sulfate p 2.5 Sodium sulfite (desiccated) 30 Hydroquinone 2.5 Sodium metaborate 10 Potassium bromide 0.5 Water to 1 liter This developer does not change the isoelectric point of the gelatin in either the developer-or the undeveloped area and since the developed gelatin image is that which is to be dyed, its isoelectric point must be kept high and the isoelectric point of the remaining gelatin must be lowered.

The films were then placed for 1 minute in the acetic acid stop bath referred to above to prevent further development and were then flashed and placed for two minutes in a pyrogallol developer of the composition stated above. This development lowered the isoelectric point of the gelatin in the area which was originally unexposed. The films were then washed and placed in a chrome alum hardener of the composition stated above for five minutes and were washed for five. minutes. The three films were then separately dyed in dye baths having a pH slightly lower than the pH of the isoelectric point of the original gelatin but above that of the isoelectric point of the gelatin developed in the pyrogallol developer.

After five minutes dyeing, the films were rinsed in an acetic acid rinse of a pH slightly lower than that of the dye but above the pH of the isoelectric point of the gelatin affected by development in the pyrogallol developer. The films were then squeegeed successively and in register to a piece of mordanted paper and the dye transferred- After complete transfer of all three dyes, a positive image was obtained on the transfer blank.

Subsequent prints can be made by redyeing and. transferring the matrices as described above.

In the methods just described, acid dyes were used and the gelatin having the higher isoelectric point was therefore dyed. It is also possible to obtain the reverse effect by the use of basic dyes, which can be made to dye the gelatin of lower isoelectric point. In both cases, the dye bath should be maintained at a pH between the pH of the isoelectric points of the two gelatin images but in the case of acid dyes, the gelatin of higher isoelectric point will be dyed and in the case of basic dyes, the gelatin of lower isoelectric point willbe dyed. Suitable'basic dyes are ,Chrysoidine (C. I. 21)Orange color Induline scarlet C. I. 827)-Red color Magenta (C. I. 67'7)Red color 7 Methyl:violet (C. I. 680 )Bl ue color Our process will now be described by reference to," the accompanying drawing wherein is depicted the modification of our method in which acid dyes are used.

As shown in the drawing, a multi-layer film I0 having three superposed emulsion layers is, used to obtain color separation images. The multilayer film III is illustrated as containing a cyan image I I in the bottom layer and for the purpose of illustration, no images are shown in the upper emulsion layers I2 and I3.

A panchromatic matrix material I4 is exposed through the multi-layer transparency In with red light to obtain exposed areas l5 in the emulslon layer. l6 of the panchromatic matrixmaterial.

, The exposed emulsion I6 is then developed in a hardening developer such as pyrogallol developer as shown in the left-hand side of the drawing to produce developed silver and hardened gelatin of lower isoelectric point in the regions ll .of the gelatin layer l6. After a stop bath and hardening treatment, the gelatin in areas I8 is hardened and a layer results which has areas of difierent isoele'ctric points. The film is then treated in an 'aciddye bath having a pH between the isoelectric points of the two images to produce dye areas 19. The residual silver and silver halide may be'removed from the film or may be allowed to remain during transfer of the dye to an imbibition blank.

In another modification of our invention as shown in the right-hand side of the drawing, the exposed film I4 is developed in a non-hardening .developer to produce developed silver in the areas 20 without differentially hardening or changing the isoelectric points of the gelatin in the two areas. The emulsion is then exposed to fog the residual silver halide and developed in a tanning developer such as pyrogallol developer to produce a silver image and gelatin of lower isoelectric point in the regions 2| of the emulsion layer. The remaining gelatin is then hardened in the originally exposed areas at 22 and the film is treated in an'acid dye bath having a pH between the pHs 0f the isoelectric points of the gelatin in the two areas. This produces a dyed image at 23in the areas'which were originally exposed and this dye -may be transferred to an imbibition blank. 7

Our process has a number of advantages over the usual imbibition process in which relief images are formed. By our method it is possible to obtain a positive matrix from a positiveoriginal in two development steps, thereby eliminating the making of separation negatives. The hot water wash is also eliminated, thereby reducing the possibility of damage and eliminating the pressure strips which cling to the edges of the film. Our

separation images may also be processed at the same time'in a single tray. It is also possible to avoid the use of an imagerestricting dye normally used in imbibition films, thereby'making possible visual registration of the matrices.

. It;Will be understood that the modification and examples included-herein are illustrative only and that our invention is to-be taken as limited only by the scope ofthe appended claims;

We claim:

1. The method of forming a colored'image in a gelatino-silver halide emulsion layer which comprises exposing the layer to a subject, developing the exposed area of said layer to form developed and undeveloped areas, hardening and lowering the isoelectric point of the gelatin in only one of said areas by development in a tanning developer, hardening the gelatin in the re-' maining area of said layer without changing its isoelectric point, and treating the layer with a solution of a dye at a pH between the pHs of the isoelectric points of the two areas to dye the gelatin of one of said areas without dyeing the gelatin in the other of said. areas.

.2. .The method of forming a colored image in a .gelatinosilverhalide emulsion layer which comprises exposing the layer toa subject, developing the exposed area in a pyrogallol developer .to lower the isoelectric point of,v and to harden the gelatin in that area, hardening the unexposed area of the layer without changing its isoelectric point, and treating the layer witha solution of an acid dye at a pH between the pHs of the isoelectric points of the two areas to dyethe gelatin of the undeveloped area of the layer but not the gelatin of the developed area of the layer. B

3. The method of forming .a-colored image'in a .gelatino-silver halide emulsion in which the gelatin has an isoelectric point at a pH of about 4.8 to 5, which comprises exposing the layer to a subject, developing the exposed area ina pyrog-allol developer to lower the isoelectric point of the gelatin to a pH of about 4.0 and simultaneously to harden the gelatin in the developed area, hardening the gelatin in the unexposed area of the layer withoutchanging its isoelectric point, and treating the layer with a solution of anacid dye at a .pH of about 4.7 to dye the gelatin of .the undeveloped area of the layer but not the gelatin of the developed area of the layer.

4. The'method of forming a colored image in a gelatino silver halide emulsion layer which-comprises exposing the layer, to a subject, developing the exposed area in a non-hardening developer, uniformly reexposing the layer and developing the silver halide of the reexposed area 'in a pyrogallol developer to lower the isoelectric point of, and to harden the gelatin in that area, hardening the remaining area of the layer with out changing its isoelectric point, and treating the layer with a solution of an acid dye at a pH between the pI-ls of the isoelectric points of the two areas to dye the gelatin of the area which was developed by the first-mentioned development step.

5. The method of forming 'a colored image in a gelatino silver halide emulsion, layer which comprises exposing the layer to a subject, developing the exposed area in a pyrogallol developer to lower the isoelectric point of, and to harden the gelatin in that area, hardening the unexposed area of the layer without 'changingits isoelectric point, and treating the layer with ai solutionof a basic dye at a pH between'thepHs ofthe isoelectric'points of the'two areas to .dyethe gelatin of the developed area of the layer.

DAVID DEAN. ROBERTO. I-IOUCK.

' REFERENCES 'CIT'ED The following references are of record in the file of this patent; UNITED STATES PATENTS Number Name a .Date

885,453 Didier Apr; '21, 1908 946,470

Spath Jan. 11,1910

OTHER REFERENCES 

1. THE METHOD OF FORMING A COLORED IMAGE IN A GELATINO-SILVER HALIDE EMULSION LAYER WHICH COMPRISES EXPOSING THE LAYER TO A SUBJECT, DEVELOPING THE EXPOSED AREA OF SAID LAYER TO FORM DEVELOPED AND UNDEVELOPED AREAS, HARDENING AND LOWERING THE ISOELECTRIC POINT OF THE GELATIN IN ONLY ONE OF SAID AREAS BY DEVELOPMENT IN A TANNING DEVELOPER, HARDENING THE GELATIN IN THE RE- 